This invention relates to a furnace and to a method of melting metal solids in a furnace, and in particular but not exclusively to a method of charging aluminum swarf in a furnace.
The charging of metal solids into a furnace in a manner which is both efficient and environmentally acceptable presents particular difficulties when the metal solids to be charged are in the form of finely divided aluminum scrap such as industrial swarf having a relatively large surface area. The surface oxide layer on the aluminium solids is resistant to incorporation of the solids into a body of molten metal and creates surface dross which must be separated and removed.
It is known to provide a holding portion of a furnace in which a body of molten metal is heated within an enclosure within which controlled combustion inhibits oxidisation of the molten metal. Metal solids are introduced in a well annexed to the holding portion of the furnace and molten metal is transferred between the holding portion and the well in order to both maintain the temperature of metal in the well and to deliver fresh metal to the holding portion.
It is also known from U.S. Pat. No. 4884786 to provide for mechanical stirring of the molten metal within the well in order to assist the rate of assimilation of solids into the liquid phase within the well. Such stirring however has the disadvantage of enhancing the rate at which molten metal within the well is oxidised by contact with air. The stirring operation is itself problematic in that the stirring implements must be formed of materials capable of withstanding the required operating temperature which materials tend to be brittle and therefore require frequent replacement.
It is also known from GB-2072819-A to avoid the need for stirring implements by supplying pumped metal to the well through a tangential channel to create a vortex and to disrupt the vortex by a further flow from radial channel entering the well at a lower level. A resulting roll-over effect is said to aid ingestion. The elevated level at which the tangential channel pours metal into the well is however likely to result in a pouring effect which will enhance the rate of oxidation. The use of alternative melting media such as molton salt reduces the rate of oxidation but has the disadvantage that the subsequent disposal of such media in an environmentally acceptable manner is costly.
It is an object of the present invention to accelerate the process of assimilation of metal solids into the liquid phase.
It is a further object of the present invention to minimise the extent to which molten metal is oxidised during the process of assimilating metal solids into the liquid phase.
It is a further object of the present invention to avoid the use of stirring implements in the charging well.